A recent investigation by the Times found adverts from a range of well-known firms and organisations had appeared alongside content from supporters of extremist groups on YouTube's video site.

An advert appearing alongside a video earns the poster about £6 for every 1,000 clicks it generates, meaning brands may have unwittingly contributed money to extremists.

The Times said that rape apologists, anti-Semites and hate preachers were among those receiving payouts.

Analysis: Rory Cellan-Jones, BBC technology correspondent

There are two difficult issues for Google here - spotting videos that are illegal and should be removed from YouTube, and determining which are legal but not suitable for advertising.

Pressed by reporters on whether the firm would employ people to actively hunt down extremist videos, Matt Brittin was evasive, indicating that a combination of smart technology and user alerts to harmful content was a better option.

But his main challenge will be in providing more clarity to customers when it comes to deciding which videos are "ad-safe".

Drawing the line won't be easy - as he pointed out, news organisations put a lot of disturbing content online but need to earn money from advertising if they are to continue to invest in journalism.

Mr Brittin seems confident that by having a thorough look at its policies and showing advertisers how they can control where their messages appear, Google can regain their confidence.

But the company, which insists it's a technology platform not a media business, is finding it ever harder to hold that line.

Media firms face tight regulation - and that is what may be coming Google's way if it doesn't clean up its act.

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Navy recruitment ads are among those believed to be affected

Last week, ministers summoned Google for talks at the Cabinet Office after imposing a temporary restriction on the government's own adverts, including for military recruitment and blood donation campaigns.

And on Monday M&S joined a growing list of firms to suspend their advertising from both Google's search engine and YouTube site. Others include McDonald's, HSBC, Lloyds, the BBC, Channel 4 and the Guardian.

'Not far enough'

Speaking at the Advertising Week Europe conference, Mr Brittin said: "I would like to apologise to our partners and advertisers who might have been affected by their ads appearing on controversial content."

He also promised that Google would exert more control over where ads were placed and improve its record for reviewing questionable content. Further details are expected later this week.

However, Labour MP Yvette Cooper told the Guardian the apology had not gone far enough.

"They need to say whether they will be paying back any of that advertising revenue and to answer our questions on what more they are doing to root out extremism or illegal activity on YouTube because they are still failing to do enough to remove illegal or hate-filled content from YouTube."

Brian Wieser, an analyst at the brokerage Pivotal, meanwhile downgraded Google's stock on Monday, saying the scandal would curtail its profits this year.

"Alphabet's Google is facing a serious issue in the UK with brand safety issues, which has global repercussions," he wrote in an analyst note.

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Media captionGoogle must "place these ads in the right way" Sir Martin Sorrell told Radio 4's The World at One

Mark Mulligan, managing director of Midia Research, told the BBC that the complaints against Google "were not new" and showed the internet was "still in its adolescence".

"When the internet was founded, it was all about doing away with the gatekeeper. But now we're facing fake news and inappropriate content and that clashes with business models like Google's which are built on selling advertising."

He said Google was likely to weather the storm, but that such incidents would weaken its dominance of the online advertising market.

"Every incident like this gives Facebook an opportunity to steal a march on Google," he said.